Depending on where you live of course. For me personally, it's way too hot in the summer because we are not used to hot weather. If you live in a country were it is most of the time very warm, then you'll love it in the summer.
Same goes for if you live in a cold weather, then I recommend autumn.
Quick tip: Don't visit the Walloon part
Spring is better, summer is best. Fall and winter are miserable in Belgium (as they are in the surrounding countries) because there's rarely any snow and it's just cold, windy and rainy.
Summers are perfect, though.
We go to Brussels about 3 times a year -
We have been lucky (weather) each time I visited in September -
I like the Xmas and New Year period, but it is often cold and wet -
And spring is often cold and wet as well -
18th June this year will be 200th anniversary of the Battle of Waterloo, which took place just outside Brussels, and decided the future of Europe for a hundred years.
Depends on what your favourite season is, it wouldn't be all that dissimilar to northern France.
9:00am on a Tuesday worked out well for me, but I was only there a short while.
